x86/build: Specify stack alignment for clang

For gcc stack alignment is configured with -mpreferred-stack-boundary=N,
clang has the option -mstack-alignment=N for that purpose. Use the same
alignment as with gcc.

If the alignment is not specified clang assumes an alignment of
16 bytes, as required by the standard ABI. However as mentioned in
d9b0cde9

 ("x86-64, gcc: Use -mpreferred-stack-boundary=3 if
supported") the standard kernel entry on x86-64 leaves the stack
on an 8-byte boundary, as a consequence clang will keep the stack
misaligned.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>